Group BY and COUNT using ActiveRecord

SQL for humans, or ActiveRecord unveiled

Under the hood, ActiveRecord translates your queries into plain SQL. Our previous GROUP and COUNT operation:

# Group therapy for SQL-phobia!
Person.group(:name).count
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Unveils this SQL equivalent:

SELECT COUNT(*) AS count_all, name AS name FROM "people" GROUP BY "people"."name"
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The name column from people is grouped, and the count of each unique name is recorded. You get a neat hash mapping names to counts.

Census with joints, or mastering complex queries

Should your operation span across a few tables, lean on ActiveRecord:

# Who needs SQL when ActiveRecord is there?
Order.joins(:customer).group('customers.name').size
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This petite yet potent line fetches the count of all orders neatly grouped by customer names. And all the join magic is performed unnoticed! The size method here mimicks count, but fits better when joining tables.

More data, more wisdom: .select() and .group()

Sometimes, more information is better:

# Why search thrice when you can get it all at once!
Person.select(:name, 'COUNT(name) as name_count').group(:name)
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Here, select allows fetching specific columns. Thus, besides the name, you get its count (as 'name_count'). The result is still an ActiveRecord::Relation, but each object carries both name and count.

One shot, all hit: Optimizing queries

Aim for efficiency in your queries. Grab all the data in a single call:

# All in one - the mantra for efficient queries!
Person.select(:name, 'COUNT(name) as name_count').joins(:articles).group(:name)
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This serves you a list of people along with the count of their articles—in a single SQL query. The output has been already sorted for any further manipulations you plan.

The lonely distinct() function

If you just want to know how common an attribute is, go distinct:

# Are you the only 'John Doe'? Let's find out!
Person.select(:name).distinct.count # => Integer
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This would return the number of distinct names—not their counts. Remember, distinct and group are NOT interchangeable!

Be a Pro with Pro-tips:

1. Scoping: Use custom ActiveRecord scopes in your Model for repeatable query patterns.
2. NULLs: NULLs are grouped together, so handle them as per your needs.
3. Having clause: Use the having method to filter grouped rows. The where clause filters before aggregation while having does after it.

Gotchas and Pitfalls:

• Data greed: Don't select or group by columns you won't use—it makes your queries slower.
• N+1 trap: You fetch a Model and then run a query for each instance, leading to lots of unnecessary queries. Use includes to load associated data in one hit.
• Misinterpreted count: count fetches the total rows—group gives count per group, and distinct counts unique values.